One known method for inspecting and analysing electronic devices such as flip-chip packaged microprocessors involves optical or infrared probing of the electronic device. During probing, a device which includes a silicon substrate: the device under test (DUT), is powered normally and its response is measured through the back of the silicon substrate by analysing the phase shift in a laser probe. For example, a modulated 1.064 μm laser probe may be used. The laser illuminates the device through an air-gap lens or a solid-immersion-lens (SIL) and the reflected beam is collected through the same lens. As the DUT is under power, it generates a large amount of heat and needs to be actively cooled to maintain its temperature and prevent it from entering thermal runaway.
In another known method, photon emission during switching is monitored. Such a method requires that the thermal conditions during the analysis are stable and reproducible in different locations. Consequently, active cooling is also required in such a test method.
Current state of the art inspection tools either use a fixed diamond heat spreader to remove heat from the device, or use liquid, such as water, sprayed on to the silicon substrate of the device to remove the heat from the device. U.S. Pat. No. 6,836,131 discloses an apparatus for spray cooling.
For example, “Transparent Heat Spreader for Backside Optical Analysis of High Power Microprocessors”, Proceedings of the 26th International Symposium for Testing and Failure Analysis (2000), pp. 547-551, to T. M. Eiles, et al., discloses a transparent heat spreader formed from a polycrystalline diamond window which is placed in contact with the die of the DUT. The DUT is viewed through the heat spreader which remains stationary relative to the DUT. Any point on the die surface of the device can be viewed through the heat spreader. U.S. Pat. No. 5,895,972 discloses an apparatus for contacting a DUT with a stationary light-transparent heat spreader. U.S. Pat. No. 6,760,223 discloses an apparatus for contacting a DUT with a stationary light-transparent heat spreader and maintaining a constant pressure.
Viewing through a diamond window is ideal for high power dissipation during optical imaging of operating semiconductors. However, viewing through the diamond heat spreader with a SIL causes a substantial loss of image quality and significantly reduces the improvement in numerical aperture (NA). Such a heat spreader can be used satisfactorily to analyse a 90 nm feature on a DUT but is incompatible with the optical characteristics of SILs required to resolve features on DUTs which have a higher resolution, e.g. features of 65 nm and below that are currently being developed. Such SILs must touch the silicon substrate of the DUT to achieve the required image resolution.
The use of a high pressure liquid to spray cool the device has limited thermal conductivity, especially near a SIL. In addition, the method does not permit wide area imaging because the turbulent liquid present in the light path impairs the image quality acquired using air-gap lenses.
Consequently, an improved apparatus for analysing semiconductor devices is required.